[6.x] Fix FilesystemManager unwanted changes

[netpok]

#30331 Introduced the following changes:

When a disk named local have no driver it thrown an exception in the previous version (through multiple levels of non-existing array keys). With this merge it will resolve to local driver.

Also it will try to create drivers for non existing disks: if filesystems.php has no disk named local the Storage::disk('local') will still try to create a local disk (actually it will fail because it tries to pass a null to a method which has an array typehint, but that's an other story).

This pull request

• restores the original working principle
• handles the root problem that made [6.x] Fix the Filesystem manager's exception on unsupported driver #30331 necessary
• Updates the test to use local disk, so a similar problem wont happen again